Medical imaging is one of the most useful diagnostic tools available in modern medicine. There are medical imaging processes of many types and for many different purposes, situations, or uses. They commonly share the ability to create an image of a bodily region of a patient, and can do so non-invasively. Examples of some common medical imaging types are nuclear medical (NM) imaging such as positron emission tomography (PET) and single photon emission computed tomography (SPECT), electron-beam X-ray computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound (US). Using these or other imaging types and associated machines, an image or series of images may be captured. Other devices may then be used to process the image in some fashion. Finally, a doctor or technician may read the image in order to provide a diagnosis.
A conventional block detector for identifying gamma events utilizes an array of photomultiplier tubes (PMTs). The array of PMTs identifies a gamma-ray scintillation event (for example, a gamma event) within a pixelated scintillation crystal block by computing the position of the incident gamma-ray from a logical combination of PMT output signals. In conventional timing readout for PET-based systems, wherein coincidence must be detected between a pair of oppositely traveling gamma-rays produced from the annihilation of a positron (that is, the gamma event), the total energy signal from the PMT array is used for signal timing purposes. Recently, silicon photomultipliers (SiPMs) have been implemented as photosensors for reading out the scintillation light of LSO (Lutetium Oxyorthosilicate) and other PET scintillators. In some implementations, the timing resolution has been achieved by coupling each single LSO crystal with typical dimensions of 3×3×20 mm3 to a single sensor pixel which is matched to the 3×3 mm2 light extraction face.
There are two fundamentally different types of SiPMs: the analog SiPM and the digital SiPM. An analog SiPM consists of an array of Geiger-mode avalanche photodiodes (APDs) (or microcells) connected in parallel to form a two terminal device. Then, although the state of the individual microcell can be described digitally as a binary state (ON or OFF), the overall output becomes an analog signal which is roughly proportional to the amount of incident light. Each gamma event leads to a complex signal shape. This shape results from the convolution of the emission characteristics of the scintillator with the temporal response characteristics of the photosensor and front-end electronics.
Typically, leading edge (LE) triggering schemes have given relatively accurate time resolutions for analog SiPMs, especially if the trigger level can be set very low to trigger on the first few photons received from the scintillator emission. However, there are limits to how low the trigger level can be set due to the presence of random dark events of the SiPM microcells and other noise sources. If the trigger level is too low, the dark rate noise can lead to a false trigger. To address this, conventional detectors have incorporated verification schemes, where only those triggers which lead to a minimum pulse height are considered for data acquisition. However, such verification schemes typically lead to detector dead time (that is, the time required for the sensor to perform the verification and for the data acquisition to be ready for the next event). In one conventional sensor, it can take at least twenty (20) nanoseconds until the sensor is ready for the next event, if the trigger is due to a dark pulse and is not verified.
On the other hand, digital SiPMs can measure the trigger time for the first, second, third, etc. avalanche of the photomultiplier to give measured arrival times for the first few measured photons. Typically, the timing resolution for digital SiPMs has been obtained by triggering on the very first avalanche. However, this has the drawback that any random dark pulse will also trigger the acquisition. Similar to the analog SiPMs, a verification scheme is needed to select the true gamma event from the dark pulses.
Thus, conventional photomultiplier sensors and schemes for acquiring data from the conventional photomultiplier sensors suffer from false triggers due to dark events and noise and detector dead time resulting from accounting for such false triggers.